This invention relates to a seal assembly for a roller cutter drill bit having a pressure balanced lubrication system, and more particularly to a seal assembly between a journal on the bit body and a roller cutter mounted for rotation on the journal.
Heretofore, seal assemblies in a rotary drill bit between the journal and roller cutter mounted thereon for rotation have included a pair of metal seal rings urged into face to face sealing contact by a pair of elastomeric seal rings which seal against the metal seal rings in addition to forcing the metal seal rings into sealing contact. Normally one of the metal rings and elastomeric rings rotates with the cutter and the other metal ring is held in a static or non-rotating position on the journal by the other elastomeric ring. Thus, sliding sealing contact is normally provided between the metal contacting faces of the opposed metal seal rings. The use of a pair of elastomeric rings permits the metal seal rings to float back and forth and move together with little chance of being separated by the severe vibrations encountered in drill bits while drilling. Any separation of the metal sealing faces permits leakage of either the drilling fluid into the bearing areas between the journal and roller cutter, or leakage of lubricant outside the bearing areas.
Metal face seals with two metal seal rings and two rings have been used for years with success to seal bearings that must operate in an abrasive environment such as, for example, track rollers for treads on tractors, such as disclosed in U.S. Pat. No. 3,180,648. A similar type of seal is also disclosed in U.S. Pat. No. 3,216,513 for use in rolling cutter assemblies for large diameter bits for mining operations such as tunneling or drilling vent shafts for mines. These seals have heretofore provided both elastomeric seal rings on the same peripheral outer surface of the metal seal rings. These mining type applications have little or no borehole pressure and consequently do not require a hydrostatic pressure compensator as used in most downhole drill bits used in oil wells. The use of a seal such as shown in U.S. Pat. No. 3,216,513 in drill bits for oil wells could have severe problems due to pressure fluctuations across the seal caused by rapid excursions of the rolling cutter on the bearing journal as the bit drills and resulting in fluid pressure differentials between lubricant inside the bit and drilling fluid outside the bit. Because both elastomeric seal rings are located on the outer peripheral surfaces of the metal rings, resulting pressure differentials could cause leakage of mud contaminants into the bearing area because the seal contact pressure of the metal rings decreases as the mud pressure becomes greater than the lubricant pressure.
A metal face seal assembly as disclosed in U.S. Pat. No. 4,516,641 dated May 14, 1985 for drill bits helps compensate for these pressure fluctuations across the seal assembly caused by axial movements of the cutter by floating movement of the rigid rings in the seal cavity to balance the lubricant volume in the space between the seal and the main bearing. As disclosed in this patent the ratio of rigid ring movement to cutter movement in an axial direction was determined to be as much as 1.88 to 1 in order to balance the lubricant in this space. This still can cause a significant pressure differential across the seal assembly as one elastomeric ring is forced to compress more while the other elastomeric ring compresses less. The reduced compression of one of the elastomeric rings also can cause the associated rigid ring to slip resulting in wear of the elastomeric seal from frictional contact with the associated metal seal. Likewise, as shown in U.S. Pat. No. 4,466,622 dated Aug. 21, 1984, a metal face seal assembly is shown including a pair of metal seal rings and a pair of associated elastomeric rings, and particularly upon movement of the roller cutter to its outermost axial position on the journal, one of the elastomeric rings has more compression than the other elastomeric ring which could result in slippage and wear of one of the elastomeric rings ultimately causing seal failure.
One of the problems involved in the wear or deterioration of bearing areas or bearing surfaces between the journal and roller cutter is the problem of the egress or entering of drilling fluid into the bearing areas. The drilling fluid normally has foreign matter or contaminates entrained therein which can be damaging to the bearing areas. In seal assemblies heretofore for roller cutter drill bits used in oil wells and requiring a hydrostatic pressure compensator which includes a pair of metal seal rings urged into face to face sealing contact by a pair of elastomeric seal rings, the elastomeric seal rings have been provided on different peripheral surfaces of the metal seal rings, i.e. one elastomeric seal has been provided on the outer peripheral surface of one metal seal ring and the other elastomeric seal has been provided on the inner peripheral surface of the other metal seal ring. Normally one elastomeric seal ring is positioned on the outer peripheral surface of the dynamic metal seal ring which rotates with the cutter while the other elastomeric seal ring is positioned on the inner peripheral surface of the static metal seal adjacent the journal as shown in the aforesaid U.S. Pat. Nos. 4,466,622 and 4,516,641. However, under certain conditions of operation, such as an axial movement of the cutter from an innermost position on the journal to the outermost position on the journal, a maximum fluid pressure differential results from the drilling fluid along with a loss of compression in one of the elastomeric rings and possible slippage and wear of that elastomeric ring. Also, a rapid back and forth movement of the seal assembly in the cavity as the cutter moves back and forth may cause violent excursions of the seal assembly from severe vibrations of the bit while drilling.